1. Field of the Invention
The present invention relates to an apparatus in which a movable member slides on a fixed member. More particularly, the present invention relates to a coating structure for the movable member. For example, a scroll type compressor has a movable scroll that slidably contacts a fixed scroll when compressing refrigerant gas between both the movable scroll and the fixed scroll. The present invention can be applied to such a compressor.
2. Description of the Related Art
Conventional scroll type compressors generally include a fixed scroll, and a movable scroll which engages the fixed scroll. When the movable scroll performs the orbital movement (referring to "revolution," hereinafter) while the spiral element of the movable scroll engages the spiral element of the fixed scroll, refrigerant gas is compressed between both the movable scroll and the fixed scroll. In recent scroll type compressors, the movable scroll revolves at high speed in order to increase the discharge capacity of the compressor. To ensure the smooth revolution of the movable scroll, the movable scroll may be formed of light materials, such as aluminum.
The wear-resistance of aluminum, however, is relatively low. Forming layers on the surfaces of both the scrolls has been proposed to improve the wear-resistance of the movable scroll made of aluminum.
Japanese Unexamined Patent Publication No. 5-86483 discloses a structure that coats the surface of the movable scroll in a scroll type compressor by use of plating. According to this structure, as shown in FIG. 8, an inner nickel-phosphorus alloy plating layer 22 containing silicon carbide particles covers the surface of the movable scroll 7 made of aluminum. The surface of the inner layer 22 is covered with an outer nickel-phosphorus alloy layer 23. The nickel-phosphorus alloy has a good adhesion to the aluminum alloy. However, due to relatively low hardness of the outer nickel-phosphorus alloy layer 23, the layer 23 has less wear-resistance.
Furthermore, Japanese Unexamined Patent Publication No. 3-92590 discloses a structure in which nickel-phosphorus alloy coats a contact surface of a fixed scroll or a movable scroll by use of chemical plating. The plated surface also has less wear resistance.